When functioning properly, the human heart maintains its own intrinsic rhythm and is capable of pumping adequate blood throughout the body's circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One manner of treating cardiac arrhythmias includes the use of a cardiac rhythm management system. Such systems may be implanted in a patient to deliver electrical pulses to the heart.
Cardiac rhythm management systems include, for example, pacemakers (also referred to as “pacers”), defibrillators (also referred to as “cardioverters”) and cardiac resynchronization therapy (“CRT”) devices. These systems use conductive leads having one or more electrodes to deliver pulsing energy to the heart. Leads are usually positioned on or in the ventricle or the atrium and the lead terminal pins are attached to a pacemaker or defibrillator which is implanted subcutaneously or in the abdomen.
Cardiac pacing leads are well known and widely employed for carrying pulse stimulation signals to the heart from a battery operated pacemaker, or other pulse generating means, as well as for monitoring, or sensing, electrical activity of the heart from a location outside of the body. Electrodes are also used to stimulate the heart in an effort to mitigate bradycardia or terminate tachycardia or other arrhythmias. In all of these applications, it is highly desirable to optimize electrical performance characteristics of the electrode/tissue interface. Such characteristics include minimizing the threshold voltage necessary to depolarize adjacent cells, maximizing the electrical pacing impedance to prolong battery life, and minimizing sensing impedance to detect intrinsic signals.
For cardiac pacemaker leads and electrodes, pacemaker implant lifetime may be partially determined by the energy delivered per pulse. Other factors that determine the energy used by the pacemaker include the electrode size, material, surface nature, and shape, the body tissue or electrolyte conductivity, and the distance separating the electrode and the excitable tissue. The pacemaker will have a longer life if the energy delivered per pulse is maintained at a minimum. The energy delivered cannot be reduced too far, however, as a critical amount of current is required for pacing. The pacing (or stimulation) threshold is a reflection of the electrical energy required for a pulse to initiate a cardiac depolarization. The saved energy can be used to provide for more features in the pacemaker. Furthermore, reducing the size of the electrode is not necessarily an optimal solution as then the electrode may be too small to properly and securely fix to the heart. Moreover, smaller electrodes may have reduced sensing capacity.
Leads are often made of some conductive metal or include a conductive metal surface coating like titanium, platinum, platinum iridium or iridium oxide. Electrical current passed through these leads, however, may cause undesired chemical reactions such as water electrolysis, oxidation of soluble species, reactions that result in the formation of gases, and metal dissolution from the electrode. The non-ideal biocompatibility of the metals used to make electrodes may therefore waste the energy provided to the electrode and result in decreased efficiency. In addition, the electric performance may be affected by the geometry of the electrode itself.
A continued need therefore exists for improved electrodes for pacing and sensing.